Cancel GPU acceleration for 2D canvas when drawing very large images

third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp

Index: third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp
diff --git a/third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp b/third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp
index e0db5591d855ae297d2134e964605ecc916ca831..e18695113413e3db4d91ea3dc405909736b4dc74 100644
--- a/third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp
+++ b/third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp
@@ -964,7 +964,6 @@ void BaseRenderingContext2D::drawImageInternal(SkCanvas* c, CanvasImageSource* i
         trackDrawCall(DrawBitmapImage, nullptr, dstRect.width(), dstRect.height());
     }
 
-
     int initialSaveCount = c->getSaveCount();
     SkPaint imagePaint = *paint;
 
@@ -1076,6 +1075,26 @@ void BaseRenderingContext2D::drawImage(ExecutionContext* executionContext, Canva
 
     validateStateStack();
 
+    // Heuristic for disabling acceleration based on anticipated texture upload overhead
+    // See comments in ExpensiveCanvasHeuristicParameters.h for explanation.
+    ImageBuffer* buffer = imageBuffer();
+    if (buffer && buffer->isAccelerated() && !imageSource->isAccelerated()) {
+        float srcArea = srcRect.width() * srcRect.height();
+        if (srcArea > ExpensiveCanvasHeuristicParameters::DrawImageTextureUploadHardSizeLimit) {
+            buffer->disableAcceleration();
+        } else if (srcArea > ExpensiveCanvasHeuristicParameters::DrawImageTextureUploadSoftSizeLimit) {
+            SkRect bounds = dstRect;
+            SkMatrix ctm = drawingCanvas()->getTotalMatrix();
+            ctm.mapRect(&bounds);
+            float dstArea = dstRect.width() * dstRect.height();
+            if (srcArea > dstArea * ExpensiveCanvasHeuristicParameters::DrawImageTextureUploadSoftSizeLimitScaleThreshold) {

[xidachen, 2016/09/23 20:56:36]

I am a little confused by this logic.

Say that srcArea = 7M*7M, so it goes into this else if block. Assuming that
dstArea is the same, then this if statement is false, and we don't disable it?
So we disable acceleration either:
1). srcArea is very large
2). srcArea is large, and dstArea is small by factor of 4

I don't quite understand the second logic.

+                buffer->disableAcceleration();
+            }
+        }
+    }
+
+    validateStateStack();
+
     // TODO(xidachen): After collecting some data, come back and prune off
     // the ones that is not needed.
     Optional<ScopedUsHistogramTimer> timer;